Transmission line measurements have been made for many years. The calculation of VSWR was first made by the simple ratio of maximum voltage, as determined by slotted line probes or sliding voltmeter, to the minimum voltage along a transmission line. Later, calibrated directional couplers permitted measurement through non-linear scale meters and an adjustable control to set the forward voltage to full scale. Several popular electronics manufacturers popularlized this system.
The scale calibration can be generated from: ##EQU1##
The development of Analog to Digital (A/D) converters for LED and LCD displays such as the National Semiconductor LM3914 gave rise to several no-knobs versions of the instrument with the forward voltage being used for the reference and the reverse voltage for the signal. While the calibration is automatic, the scale is still non-linear. Obviously, then, it would be desirable to provide an improved VSWR meter with a linear scale.